The use of slack adjusters to compensate for "play" or looseness occurring in reciprocating mechanisms due to gradual wear of parts or other causes, long has been known. Early examples of such devices may be seen from U.S. Pat. Nos. 893,220, issued July 14, 1908, 1,328,341, issued Jan. 20, 1920, and 1,796,009, issued Mar. 10, 1931. One of the most obvious applications for slack adjusters of the automatic type in the power tranasmitting mechanism of vehicle brakes, since the gradual wear of brake friction members is an inherent condition causing slack in brake devices. Examples of slack adjusters in vehicle brake mechanisms may be seen from the teachings of U.S. Pat. Nos. 1,509,225, issued Sept. 23, 1924, 2,792,085, issued May 14, 1957, and 3,511,103, issued May 12, 1970.
The last of these patents, issued in the name of the inventor of this invention, is illustrative of certain shortcomings in the state of the slack adjuster art that have prevented the wide commercial acceptance of certian devices including slack adjusters. This patent discloses a so-called "wedge" brake mechanism whereby braking is actuated by the linear movement of a wedge between two force transmitting assemblies, causing opposite movement of these assemblies and movement of the brake shoes, driven by the assemblies, into braking positions. Although it generally is recognized by experts in the vehicle brake field that brakes utilizing the wedge principal possess many advantages over other types of brakes in numerous usages, commercial acceptance of wedge brakes has been less than would seem justified by these advantages. It is believed that a primary reason for this is the heretofore unavailability of a wholly satisfactory slack adjuster for inclusion in wedge brake assemblies, as well as other usages where the slack adjuster must function automatically in a relatively maintenance free manner in an environment extremely hostile to mechanical devices, and yet be easily adjusted manually during maintenance.
For example, the slack adjuster illustrated in the above mentioned U.S. Pat. No. 3,511,103 permits threads formed on its output member to be exposed to the normal atmosphere in which vehicle brakes function, an atmosphere including road grime and corrosion promoting chemicals such as salt. While an attempt was made to remedy this exposure by the disclosure of U.S. Pat. No. 3,797,613, issued Mar. 19, 1974, by use of an elastomeric boot to protect adjuster threads, it readily may be appreciated that any failure or malfunction of the boot under vehicle operating conditions would expose the threads of the slack adjuster to possible damage. While the prior art has included slack adjusters wherein vulnerable operating elements such as threads are not exposed or potentially exposed to ambient conditions (as disclosed by above mentioned U.S. Pat. No. 1,509,225), it is believed such arrangements have been disclosed only in configurations of relatively large axial lengths compared to thread diameters. These configurations could not be used easily in applications such as vehicle brakes where packaging space for slack adjusters is quite limited, but relatively large forces must be generated to accomplish the desired adjustment.
It is, therefore, an object of the present invention to provide an automatic adjuster in which the vulnerable working parts completely are sealed from the ambient atmosphere, and may in fact be surrounded by an anti-corrosion, lubricating medium. A further object of this invention is to provide such a slack adjuster that easily may be adjusted manually when desired. A still further object of the invention is the provision of such a slack adjuster that has few and simple working parts such that it is extremely compact in size, simple to assemble and reliable in performance.